Oh man what a cutie!
I agree that the Walmart seed mix is definitely not the best, but the Volkman parrotlet blend is good if your guy won't take to pellets. If you do go with only seed, one really great way to boost it's nutritional value is by sprouting! Sprouting reduces the fat, increased the protein, and boosts the levels of key nutrients in seed. Since most seed loving birds will accept it right away, it's a great way to make sure your bird is getting enough fresh food.
Just to give you an idea of what I feed my parrotlet: 1 tbsp "chop", 1 tsp pellets, 1 tsp fresh sprouts, and 1 tsp "other" (e.g. millet, Green Chunks, cooked food mix, Nutriberries, etc).
Variety is probably the most important part of a parrots diet so offering an assortment of different foods along with his seed will ensure he has a balance diet.